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Double wishbone setup

olberj - 3/1/16 at 10:55 PM

I'm looking to upgrade the Bonito suspension to double wishbones all round when i redesign the chassis for the v8.

Is it worth trying to make an off the shelf kit work, if so, what's recommended?

Or should i go ahead and build my own from scratch?

I'm going to have to make custom driveshafts anyways so the uprights n hubs can be whatever suits best.

Camber Dave - 4/1/16 at 09:10 AM

If you are thinking of doing your own

Design the rear first

Find an off the shelf upright and wheels (it’s always cheaper than building your own) Or to DIY, see the Haynes book.
Use the driveshafts that come with them (if you can’t use the donor diff get the shafts mated to your inboard flanges) this sets the track width.
From the upright set the lower arm to rise no more than 1 deg to the chassis. Set the chassis brackets as wide as practical.
Set the upper arm to ¾ to 7/8 of the length of the lower arm and falling by 3 to 4 degrees to the chassis
Verify the geometry with a computer programme. Search on here for links to the programme.

Front

Select an upright…. A long thread in itself – Search on here, most options have been covered before.
If you are using different wheels it could have a different stud pattern if it gives better geometry.

Select a track width 20 to 40 mm wider than the rear.
From the upright set the lower arm to rise no more than 1 deg to the chassis. Set the chassis brackets as wide as practical.
Set the upper arm to ¾ of the length of the lower arm and falling by 4 to 5 degrees to the chassis
Again verify the geometry with a computer programme.
Ideally plan the entire design on a computer or drawing board. It will save hours of wasted parts and material in the garage.

Design the coilover to be as upright, and its mounting to the lower arm as close to the wheel as practical. Usually it will foul the upper arm! so a compromise will result.

Neville Jones - 4/1/16 at 11:28 AM

Early MX5 uprights, front and back, would seem the straightforward choice.

Plenty of them about, and cheap.

Also a lot of info about on mounting them to wishbones.

Cheers,
Nev.

Sam_68 - 4/1/16 at 01:07 PM

Can of worms, this one, I know.

Everybody has their own approach, but for what it's worth:

If it's to fit an existing chassis/bodyshell, start by carefully measuring that, and draw up the constraints it imposes (which will include track dimensions, I assume?).
Next most critical thing, in my opinion, is to fix the roll centre positions. Roll axis will usually slope up slightly from front to rear, and IMO roll centre locations should be closely fixed, relative to the sprung mass (chassis), through the full range of suspension movement (important so that you get predictable weight transfer).
Personally, I'd then work out the FRONT suspension first, as you are usually more constrained by the geometry of the front uprights (fabricated rear uprights are easy, if push comes to shove) and steering, whereas you usually have a bit more freedom at the rear.
Don't get too hung up on 'rules' about the angles of wishbones (particularly the oft-met myth that lower front wishbone should be horizontal at normal ride height). They're OK as rules-of-thumb and as a starting point, but don't cling to them to the extent of preventing other objectives being met.
Remember at all times that the car has two ends. It may sound facetious, but you'd be surprised how many people ignore the fact that the roll centre location and wheel cambers at one end of the car need to work in relation to what's happening at the other end of the car, and that the two ends of the car are connected by a (relatively) rigid chassis, so can't behave independently of each other. You want to be aiming at a degree of progressive understeer (ie. the grip at the front getting progressively slightly worse than that at the back, the more the car rolls).

...and when you've got your head around all that, remember that the geometry is only one half of the picture. The other, more complex part is the springs and dampers: for example, you'll find that the car doesn't neatly roll around the geometric roll axis like a pig on a spit, because the springs, dampers and ARBs at either end of the car resist it to different degrees, and in any event you're seldom interested in steady-state cornering. So all your carefully-calculated camber angles under roll are pretty much nonsense, anyway.

But you've got to start somewhere...

If any of the above fails to make sense, get a copy of Staniforth's 'Competition Car Suspension', read and understand it (particularly the chapter on weight transfer), before going any further: even if you end up with a design that's dictated by your choice of uprights and the existing chassis, at least you'll be able to make decisions that are somewhat informed.

olberj - 4/1/16 at 07:42 PM

Brilliant folk. I very much apprecite the input.

Bonito currently has cortina uprights. I have the a8 front uprights and shafts too which i can use and the rears too, it's quattro, but the subframes etc are all far too large and comfort based.

That's why I'm thinking using purpose built uprights to start with. Mx5 might be a goer but unsure if the cvs will handle the v8 power n torque.

AdamR20 - 4/1/16 at 10:46 PM

Short reply because typing on a phone is a pain... but... I think a few people have fitted V8s to MX-5s with standard rear ends, and check out www.vsusp.com

Neville Jones - 5/1/16 at 10:43 AM

Depending on the V8, but 1.8 MX5 rears can handle a good amount of abuse. If it's a yank 8, then you could end up marginal, but the yanks seem to keep their Chevy powered MX's in one piece.

All the garbage above about suspension design can be sidestepped, and just follow Staniforth, and you'll be fairly safe.

And fwiw, a non horizontal bottom wishbone at normal ride ht, can introduce some unwanted behaviour. Best to stick with accepted norms, unless you're an F1 designer building the next generation.

Cheers,
Nev.

olberj - 5/1/16 at 11:12 AM

It's an Audi v8 4.2 so circa 300bhp.

olberj - 5/1/16 at 11:17 AM

Would reverse engineering, with a jig on the chassis to keep the wheel centres and outer disc face in place, translate to double wishbones ok?

No idea if the original design was goid but it was intended for racing.

Sam_68 - 5/1/16 at 11:53 AM

quote:
Originally posted by olberj
Would reverse engineering, with a jig on the chassis to keep the wheel centres and outer disc face in place, translate to double wishbones ok?

No idea if the original design was goid but it was intended for racing.

I'm guessing you might end up having to make substantial alterations to the back end of the chassis to get frame members where you need to place wishbone pickups and diff mounts. Try comparing the live axle and IRS versions of various 'Seven' type kit cars to see what's involved - you you generally find that there are pretty significant alterations to everything behind the rear cockpit bulkhead. Although the rear of the chassis on the Bonito doesn't look a bad, all things considered... the framework that supports the upper crossmember for the coilovers looks like a fair starting point to graft on additional tubes.

It does beg the question of whether it's worth the effort if your existing 5-link live axle set-up is reasonable, though.

Your front suspension is standard Cortina (including wishbones) at the moment, I guess?

olberj - 5/1/16 at 12:26 PM

The front is indeed.

The rear is being built from scratch around the engine and gearbox. May have omitted it's going mid engined, sorry.

We will add a cage to the tub and work backwards from there so everything behind firewall will be scratch built.

Camber Dave - 5/1/16 at 01:19 PM

I am also a great advocate of the great Alan Staniforth and agree with most of the other posts including Sam.

The starting parameters above were derived from hundreds of chassis analyses and rectification’s.
They will establish basic stable Roll Centres/roll axis, that WILL need to be refined with software as advised.

MX5 Front uprights are an improvement over the Cortina ones because they give an improved Scrub radius.
However the Rear ones are cranked inboard reducing the top wishbone length. They also have a reputation for having the shaft seizing solid into the flange.

The weight transfer is a function of Height of the Cof G, track width and stable Roll axis.
It is tuned by roll resistance: ie spring rates and roll bars, which by definition will be added and amended AFTER the basic chassis is mostly finished.
Errors in geometry and roll centres are more difficult to fix on a completed chassis.

Sam_68 - 5/1/16 at 02:08 PM

Sorry, Olber - yes I failed to appreciate that you were converting to a mid-rear engine installation. That obviously makes a bit of a difference!

quote:
Originally posted by Camber Dave
The weight transfer is... tuned by roll resistance: ie spring rates and roll bars, which by definition will be added and amended AFTER the basic chassis is mostly finished.

Off topic...

That's most certainly the traditional way of doing things, but with recent advances in computer analysis (though I stress that we're talking about stuff that can be achieved with an Excel spreadsheet; nothing too fancy), there is an alternative possibility of integrating the weight transfer and the kinematics much more fully from the outset and even, perhaps, (whisper it!) considering weight transfer first.

After all, most people have a pretty good idea from the outset of what suspension frequencies they want to work with, and from there and your estimated weight distribution, you can reverse engineer the roll axis and kinematics.

The potential advantage of doing it this 'wrong way round' is that you can rely less on ARB's and dampers (both of which have nasty side-effects) to sort out any imbalance.

Of course, the 'rules of thumb' are based on years of experience, so you'll probably end up near enough the same place, whichever way around you do it, but there's no harm in trying to understand why you're doing what you're doing, instead of just blindly following others' advice.

On topic (more or less)...

The practical corollary of all this for the OP is that the kinematics and weight transfer (including roll axis) for a front-engined car like an MX5 or the 'standard' Bonito will be a bit different from a mid-rear engined car with a dirty great V8 in the back.

So just copying MX5 geometry (for example) might not be the best solution.

On that basis, personally I might be inclined to keep the front, Cortina-based geometry, if t works OK (you can use alloy replacement uprights and tubular wishbones, if you like), to avoid having to mess around with the chassis pickup positions, steering, etc., then design the rear geometry to work with the front.

alfas - 7/1/16 at 11:01 PM

maybe some inspirations ...found here, in the locost forum:

http://www.locostbuilders.co.uk/viewthread.php?tid=199415

some other info´s:

http://dragracing.de/showthread.php?5562-V8-Bonito-Projektstart/page2&s=6c12d427772978e28c11a6fec2e1ccbd

https://www.facebook.com/media/set/?set=a.971763642858464.1073741869.145729158795254&type=3

olberj - 7/1/16 at 11:20 PM

Thanks. Yeah that first link is me.

alfas - 7/1/16 at 11:32 PM

oh...now i see thats you....

but why you dont finish the car with the acutal chassis...as this looks well made and its already prepared for a bigger (front) engine. why changing to mid-engine design?

now on second view, the basic layout and shape reminds me a lot on the TVR chassis (besides TVR is using round tube and has IRS instdead the live axle)...tvr wedges even used the cortina double wishbone

have you ever though to put your bonito body on a TVR chassis...ok, again not mid-engine, but a well desinged suspension layout.

olberj - 7/1/16 at 11:39 PM

Thanks for the links. They are great.

I have a real Ford GT sat in the workshop for inspiration. Every day i see it and know that's the only way to fly!

Did consider a front engine v8 but it was either rover v8 or a lot of reworking the front end so this came up and we thought we best give it a go.

Those links are a huge help. Cheers.